Please note addition to para. 2 whereby ENGINES MUST BE EXPOSED.

OK Mike, I think I understand what you REALLY intend with this.  But for the true purist scale modeler who is a word smith that might get hung up on what is allowed in Class C contests, you might want to more clearly define what is meant by an engine that "MUST BE EXPOSED".    I am hoping that an engine that has its cylinder "exposed" is compliant with your rule.  If the engines have mounting lugs (Norvels and Wasps and Cox TD's), if they are visible from some perspective, I am hoping that such will be compliant with your rule. 

But if words are used that just the cylinder must be exposed, what is defined as a cylinder and what part of the crankcase is not the cylinder.  The reason I ask is that I have several projects lined up that have engine nacelles that are less than 1.5 inch wide and/or diameter.  That means that the portion of the engine that will be inside that 1.5 inch diameter would not necessarily be "EXPOSED" but would still be compliant with the allowed maximum cowl width of 1.5 inches.  ((In fact, I already have a 1/2A multi engine scale model where the nose on one of the engines is only 1.5 inches wide, the engine is side mounted, and from the side, the entire crankcase/cylinder and mounting lugs are visible, but part of the nose fairing over the engine as in a cowl (and within the 1.5" allowed maximum) covers the mounting lugs and a portion of the upper part of the crankcase).

Another situation for consideration is that given the size and power of these engines that can be used for 1/2A multi-engine scale, a scale subject could be chosen and built to a scale, particularly an aircraft with large radial engines, where the scale model would have say a 3 inch diameter cowl and could possibly house an engine with only the top fins of the glow head "exposed".  From the front of the cowl, however, the engine could be considered to be completely exposed.

And is there a need to address if the bottom side of an engine crankcase must also be "exposed"?

"Exposed" is an interesting word.

bared, unprotected, uncovered, visible, showing, open, wide-open, out in the open, open to the elements

Just wondering.

Keith
(An interested 1/2A multi-engine profile scale enthusiast)

Having thought it over, just disregard that line about "engines must be exposed".  Too much confusion there.